Some buildings utilize air purification systems to remove airborne substances such as benzene, formaldehyde, and other contaminants from the air supply. Some of these purification systems include photocatalytic reactors that utilize a substrate or cartridge containing a photocatalyst oxide. When placed under an appropriate light source, typically a UV light source, the photocatalyst oxide interacts with airborne water molecules to form hydroxyl radicals or other active species. The hydroxyl radicals then attack the contaminants and initiate an oxidation reaction that converts the contaminants into less harmful compounds, such as water and carbon dioxide. It is further believed that the combination of water vapor, suitably energetic photons, and a photocatalyst also generates an active oxygen agent like hydrogen peroxide as suggested by W. Kubo and T. Tatsuma, 20 Analytical Sciences 591-93 (2004).
A commonly used UV photocatalyst is titanium dioxide (TiO2), otherwise referred to as titania. Degussa P25 titania and tungsten dioxide grafted titania catalysts (such as tungsten oxide on P25) have been found to be especially effective at removing organic contaminants under UV light sources. See, U.S. Pat. No. 7,255,831 “Tungsten Oxide/Titanium Dioxide Photocatalyst for Improving Indoor Air Quality” by Wei et al.
A problem with air purification systems using UV-PCO technology has arisen. Currently available systems exhibit a significant loss in catalytic ability over time. This loss of catalytic ability has been at least partially attributed to volatile silicon-containing compounds (VSCCs), such as certain siloxanes, present in the air.
The aggregate amount of volatile organic compounds (VOCs) in air is typically on the order of 1 part per million by volume. In contrast, VSCC concentrations are typically two or more orders of magnitude lower. These VSCCs arise primarily from the use of certain personal care products, such as deodorants, shampoos and the like, or certain cleaning products or dry cleaning fluids, although they can also arise from the use of room temperature vulcanizing (RTV) silicone caulks, adhesives, lubricants, and the like. When these silicon-containing compounds are oxidized on the photocatalyst of a UV-PCO system, they form relatively non-volatile compounds containing silicon and oxygen that may deactivate the photocatalyst. Examples of non-volatile compounds of silicon and oxygen include silicon dioxide, silicon oxide hydroxide, silicon hydroxide, high order polysiloxanes, and the like. These compounds may be at least partially hydrated or hydroxylated when water vapor is present. Increasing the catalyst surface area alone does not necessarily slow the rate of deactivation as might be expected if the deactivation occurred by direct physical blockage of the active sites by the resultant non-volatile compound containing silicon and oxygen.
There is a need for improved UV-PCO systems that can aid in the elimination of fluid borne contaminants in a fluid purifier and can operate effectively in the presence of typically encountered levels of VSCCs such as siloxanes.
Literature data indicates that TiO2 and ZnO can generate gaseous oxidants, possibly hydroxyl radicals (.OH) and hydrogen peroxide radicals (.OOH), but most likely hydrogen peroxide (H2O2). These volatile oxidants can travel up to 100-500 μm and be converted, if necessary, to .OH by H2O2 photolysis. The resulting now hydroxyl species, generated at some distance from the original TiO2 photocatalyst surface, can oxidatively destroy organic compounds or films which are not in contact with TiO2, hence the name “remote” photocatalyzed oxidation. Example film materials oxidized include palmitic acid multilayers, soot particles, and absorbed organic dyes (methylene blue).
Such remote oxidation has recently been demonstrated for organo-silicon compounds including octadecyl-triethoxysilane (ODS), (Tatsuma et al. (2002)) and related silanes: heptadecafluoro-decatrimethoxysilane, octodecyltriethoxysilane, and methyltriethoxysilane. Presumptively, such Si-containing compounds will leave a silica (SiO2) residue, which is expected to be similar to the “silica” produced by photocatalysts due to adsorption and degradation of organo-siloxanes from cosmetic products, etc.